A chemical sensor typically includes a vibrating membrane having a polymer coating that is capable of being loaded with vapors and interferents. As the vapor loading takes place, the mass carried by the membrane increases causing a shift in its vibration frequency. The frequency shift is a function of the type and amount of vapors loaded. In general, more than one vapor can load onto each polymer coating, but some coatings have a stronger affinity for particular vapors.
Normally, an array of sensors, each having a distinctive polymer coating, is used together; the vibration frequency of each membrane is measured and recorded. An array is usually selected to optimize a certain performance criterion of interest, such as maximizing the probability of detecting a lethal vapor of interest or minimizing the probability of false alarms. Signals representative of the vibration frequencies of an array of sensors can be interpreted to determine concentration levels of vapors present in the atmosphere.
The selection of an appropriate array has been hampered by the scanty information available on the response of coatings to vapors, and the reliability of that information. Most of the available information is based on experiments conducted on single vapor/coating pairs. These experiments are time-consuming. Additionally, processes do not exist for using the information derived from these experiments to reliably extrapolate the response of the coating to the vapor at concentration levels outside of the experimental range. Processes also do not exist for using the information derived from these experiments to reliably extrapolate the response of the coating to multiple vapors. Moreover, it is also difficult to store the information derived from these experiments efficiently. Some of the information on the reaction of coatings to vapors is based on linear models. This information may not be reliable because linear models do not adequately describe the loading process. In particular, linear models fail to account for the saturation phenomenon that relates to the finite capacity of a coating to respond to one of more vapors. As a result, the traditional processes and apparatus for estimating the concentration of a vapor in the environment do not provide an acceptable performance level.